When the Telegraph's Charles Spencer reviewed Nicole Kidman in David Hare's The Blue Room, he described the experience, notoriously, as "pure theatrical Viagra". At the time, the world's flesh crept, then we moved on. But Spencer's not-so-bons mots come to mind today, when news breaks of an even randier conjunction of sex and the stalls. In interviews on The Graham Norton Show and at the Hay festival, actors Alex Kingston and Rob Lowe have gone public on a sensation sweeping theatreland: a mystery couple on a mission, apparently, to have sex in every playhouse in the West End.

Kingston and Lowe both claim to have sighted the couple from the stage, in flagrante delicto, while the actors struggled to stay on script. Lowe was muddling through an adaptation of Aaron Sorkin's A Few Good Men when his attention was grabbed by carnal antics in the royal box. "They were directly in my eyeline," he told an audience at Hay, "and at it in the middle of the show." According to the Daily Mail, singer-turned-actor Michelle Williams claims to have spotted (the same?) couple at play from the stage of the musical Chicago two years ago. "There was this woman sat there with her arm over the chair," Williams said, "getting her kicks in the front row with her boyfriend." (The front row? Did nobody make a harrumphing noise?)

What, then, are we to make of this libidinous pair, working so impressively to put the shaft, and indeed the "'avin' you", back in Shaftesbury Avenue? You can see why they chose Chicago. Its marketing campaign – all fishnets and flirtation – is all about sex: our randy heroes just took the imagery to its logical conclusion. But sexy plays are not exactly thick on the ground, as the couple's choice of A Few Good Men implies. When theatre does sex, it's apt to be a searing critique of lovelessness or depravity. Good luck to any couple planning a steamy night along to Anthony Nielson's Stitching or Mark Ravenhill's Shopping and Fucking. A lifetime of celibacy would surely follow.

Good luck, too, to those attempting a bunk-up in a traditional West End theatre, where leg room is scant and leg-over room surely nonexistent. In her Graham Norton interview, Kingston challenged the pair to try it on at the intimate Donmar Warehouse, where she'll appear in Schiller's Luise Miller. But more experimental venues – and shows – may make for happier humping ground. The nooks, crannies and darkened corners of Punchdrunk's Masque of the Red Death; Duckie's upcoming Lullaby, whose audiences stay overnight; Ontroerend Goed's one-on-one speed-dating show Internal – these productions are made for clandestine (or, in the last's case, more or less overt) sex. Otherwise, the royal box is your best bet. Having had one to myself a few weeks ago at Clybourne Park, I now realise – too late! too late! – that I could have been orgying from the waist down and the cast would have been none the wiser.

What such an act would achieve remains a mystery. Are the couple in question theatre fetishists, aroused (as are all theatre fans, to a more restrained extent) by the frisson of live performance? Are they adrenaline junkies turned on by the thrill of the illicit? Or is this a sophisticated form of theatre criticism, a sort of copulative one-star review? Rob Lowe's confidence, as well as his concentration, must have been knocked when he spotted our heroes in the royal box. There are several reasons why a couple might have sex in a theatre, but absorption in the events onstage is unlikely to be one of them.